A hetero-junction bipolar transistor (hereafter referred to as HBT in some cases) is a bipolar transistor wherein the emitter-base junction is made a hetero-junction using a substance having a larger band gap than the base layer for the emitter layer in order to enhance the emitter injection efficiency, and is suitable as a semiconductor element used in the frequency region of the microwave band or higher.
For example, in the case of a GaAs HBT, a compound semiconductor epitaxial substrate wherein a p-n junction, which is an emitter-base junction, is the structure of the hetero-junction, is generally produced by sequentially crystal-growing an n+-GaAs layer (sub-collector layer), an n-GaAs layer (collector layer), a p-GaAs layer (base layer), an n-AlGaAs layer or an n-InGaP layer (emitter layer), and an n-GaAs layer (sub-emitter layer) on a semi-insulating GaAs substrate, using a metal organic chemical vapor deposition process (MOCVD process). HBT is manufactured using the compound semiconductor epitaxial substrate. In addition to these, a compound semiconductor epitaxial substrate for an HBT using an InP substrate is also widely used.
FIG. 3 is a diagram showing the pattern of the layer structure of a conventional commonly used GaAs HBT. In the HBT 100, a sub-collector layer 102 consisting of n+-GaAs, a collector layer 103 consisting of n-GaAs, a base layer 104 consisting of p-GaAs, an emitter layer 105 consisting of n-InGaP or n-AlGaAs, a sub-emitter layer 106 consisting of n+-GaAs, and an emitter contact layer 107 consisting of n+-InGaAs are formed in this order as semiconductor thin film crystalline layers on a semi-insulating GaAs substrate 101 using an appropriate vapor phase growing process, such as an MOCVD process. A collector electrode 108 is formed on the sub-collector layer 102, base electrodes 109 are formed on the base layer 104, and an emitter electrode 110 is formed on the emitter contact layer 107.
In thus constituted HBT, the current gain β is represented by the equation:β=Ic/Ib=(In−Ir)/(Ip+Is+Ir)wherein Ic represents a collector current, Ib represents a base current, In represents an electron injecting current from the emitter to the base, Ip represents a hole injecting current from the base to the emitter, Is represents an emitter/base interface recombination current, and Ir represents a recombination current in the base.
Therefore, in order to enhance the current gain β, from the above equation, the recombination current Ir in the base must be reduced. Although the recombination current in the base is significantly affected by the crystallinity of the base layer, it is also significantly affected by the crystallinity of the sub-collector layer, and the crystallinity of the substrate used for growing. With the lowering of these crystallinities, the current gain β is also lowered.
Therefore, the current gain β of the HBT functional layers formed on the substrate is affected by the dislocation density of the substrate, there is a problem in that the fluctuation of electrical characteristics of the HBT is caused by the kind of the substrate to be used even if the same manufacturing process is applied. In order to solve this problem, methods for forming a buffer layer on the substrate have been heretofore proposed. For example, in the case of a GaAs substrate, there has been proposed a constitution wherein a GaAs or AlGaAs lattice structure, or a GaAs/AlGaAs super-lattice structure is formed in the buffer layer, thereby preventing the propagation of defects present in the substrate into the epitaxial layer formed on the substrate.
However, by the constitution wherein the above-described lattice structure or super-lattice structure is formed in the buffer layer, the current gain characteristics still fluctuate depending on the kind of the substrate to be used, probably because the propagation of defects in the substrate into the epitaxial layer formed thereon cannot be sufficiently suppressed. Therefore, a compound semiconductor epitaxial substrate that can obtain stable electrical characteristics in manufacturing not depending on the fluctuation by lots, the crystallinity or the like of the substrate has been demanded. It is also demanded to manufacture a device having favorable current gain characteristics without being affected by the defects in the substrate.